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An instrument that converts the energy of sound waves into electrical signals is a microphone. When a person speaks into a microphone, sound waves strike a flexible diaphragm and cause it to vibrate in the pattern of the sound waves. The diaphragm is connected to an electric circuit in such a way that the movement of the diaphragm causes a corresponding change in the circuit, causing an electric current to flow. The strength of the current is proportional to the pressure applied to the diaphragm. Because the microphone is converting one type of energy (sound waves) into another (an electrical signal), it is known as a transducer. A loudspeaker performs the same process in reverse.